In the previous art, electronic air filters of the charge media type, a filter medium is positioned between metal screens and polarized by applying high voltage between these screens. In many cases, the high voltage supply is an integral part of the filter. Examples are my U.S. Pat. No. 4,549,887 and U.S. Pat. No. 4,828,586. These inventions effectively describe filters which have two outside grounded screens and an inside screen which is charged with high voltage. Between the screens there are pads of dielectric fibrous trapping material which becomes polarized by the electric field between the screens.
These filters are usually one or two inches thick and they get their power from a low voltage supply such as 24 volts which is usually available for the air-handling units. These filters use very little power, about 1½ watts to 2 watts. Their electronic system converts a low voltage input to high voltage, eg, approximately 7 KV to 12 KV. The high voltage creates an electrostatic field inside the filter and polarizes the fibrous media which then better attracts the dust from the air flow. The method of attaching the power supply electronics to the filter itself is one object of this invention.
The amount of voltage which can be applied between these screens is limited by the space between the screens. Typically for a one-inch thick filter, the applied voltage is approximately 7 kilovolts. If the voltage is increased beyond this, avalanche arcing has in the past occurred between the inside screen and the outside grounded screens. This produces a loud sparking noise. Avalanche discharge occurs when a small leakage starts which ionizes the air and generates a conductive path between the screens at one spot. This causes the charge on the inside screen to dissipate abruptly thus making the loud noise. The effect is intense because the inside screen and the outside screens form a capacitor with the dielectric media being the dielectric.
U.S. Pat. No. 5,573,577, by the same inventor, describes a similar filter where conductive strings are used in place of the inside screen. The purpose of using the strings is to provide internal ionization via the loose ends of the fibers. These strings feature loose fiber ends and they are rendered conductive by some means. In this case, avalanche discharge is very minimal because the strings, by their small total surface have very small capacitance. In practice, they are about 1¼ inches apart. The actual area covered by the strings is much smaller as compared to the area covered by an equivalent screen. This is why the strings have very small capacitance.
Another object of my invention is therefore to provide a method of allowing a higher voltage to be employed without the presence of severe avalanche discharge.
By reducing avalanche discharge in these filters and enabling application of higher voltage between the screens, a higher efficiency of the filter is provided.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.